1. Field of the Invention
The present invention relates to a liquid-crystal display device of optical writing type.
2. Description of the Related Art
The inventors of the present invention know that a liquid-crystal display device of optical writing type is typically used as a liquid-crystal light valve.
Such the liquid-crystal display device of optical writing type includes two transparent electrodes located in opposition to each other. On the first transparent electrode, there is formed a photoconductive layer made of hydrogenated amorphous silicon (a-Si:H). On the photoconductive layer, there is formed a light-absorbing layer made of a metal thin film such as a C (carbon) film or an Ag (silver) film, and so on. On the light-absorbing layer, there is formed a dielectric mirror layer composed of a multi-layer film made of TiO.sub.2 (titanium dioxide) and SiO.sub.2 (silicon dioxde), of ZnS (zinc sulfide) and MgF (magnesium fluoride), and so on. Between the dielectric mirror layer and the second transparent electrode, liquid-crystal layer consisted of liquid-crystal elements is closed through an orientation film.
The dielectric mirror layer is arranged so as to reflect a projected ray of light for reading an image data written in the liquid-crystal elements of the liquid-crystal layer so that the projected ray of light comes out from the liquid-crystal elements, and to reflect the projected ray of light with a high reflectance for preventing the reflected ray of light from an incidence to the photoconductive layer. The light-absorbing layer is arranged so as to prevent a ray of light used for writing an image data from a re-reflection against the photoconductive layer, and to prevent the projected ray of light for reading out passed through the dielectric mirror layer.
As will be understood from the above, the projected ray of light for reading out, which is passed through the liquid-crystal layer, is reflected on the dielectric mirror layer, and then outgoes from the liquid-crystal display device. However, the dielectric mirror layer has a reflectance of about 95%. It means that about 5% of the projected ray of light is allowed to pass through the dielectric mirror layer. When the ray of light passed through the dielectric mirror layer is incident to the overall surface of the photoconductive layer, the image information written by the ray of light for writing will disappear. In order to prevent this shortcoming, the light-absorbing layer is provided between the dielectric mirror layer and the photoconductive layer. The ray of light incident on the liquid-crystal layer has an intensity of about 1 W. On the other hand, in case the photoconductive layer is formed of hydrogenated amorphous silicon (a-Si:H), this photoconductive layer changes its conductivity in response to the ray of light having an intensity of several tens .mu. W or more. In this case, hence, it is necessary to reduce the intensity value of the ray of light incident to the photoconductive layer by four to five digits by means of the dielectric mirror layer and the light-absorbing layer.
For this purpose, it is possible to take one measure for enhancing a reflectance of the dielectric mirror layer. As a result, the dielectric mirror layer is required to have more laminated dielectric mirrors. With more dielectric mirrors being laminated, the dielectric mirror layer keeps thicker and thicker, resulting in bringing about difficulty in applying a sufficient voltage to the liquid-crystal layer.
In order to cope with that shortcoming, the foregoing liquid-crystal display device of optical writing type has the light-absorbing layer composed of a metal thin film such as a carbon film or a silver film for the purpose of lowering the intensity value of light by four digits or more.
In case of using as the light-absorbing layer the metal thin film such as carbon or silver, the light-absorbing layer has a high conductivity. As a result, the photoconductive layer keeps a higher conductivity. Hence, it is necessary to keep the metal thin film patterned in the liquid-crystal display device. As a result of patterning the metal thin film, the manufacturing process becomes technically difficult for obtaining high resolution. Moreover, since inferior adhesion appears between the metal film such as a carbon film or a silver film and hydrogenated amorphous silicon (a-Si:H), the light-absorbing layer is likely to be stripped off the photoconductive layer.